An organization which places a large number of long distance telephone calls can reduce their long distance charges by directing the calls through appropriate long distance carrier. For example, many long distance carriers have charge schedules wherein the cost of all calls made on a particular WATTS line during a predetermined period are reduced by a certain amount when a number of calls exceeds a preset level. There has arisen a demand for telephone switching systems which can economically control the routing of local telephone calls (which may be received through a local telephone company) to appropriate long distance carrier lines.
The above type of telephone switching system may include a large number of local voice signal lines that are each connected to a separate input codec (coder/decoder). Each input codec samples its input at a different instant during a cycle, and transmits a corresponding digital signal over a bus to a crosspoint circuit. From the crosspoint circuit, the signals travel to another bus to each of a large number of output codecs that can each sample the bus at an appropriate instant during the cycle, convert the digital sample to an analog voice signal, and deliver the voice signal to a chosen line of a chosen long distance carrier. In order to allow a digital byte or frame generated by an input codec, to be picked up by the appropriate output codec, both codecs must be activated at precisely the same time in each cycle. A different synchronizing line is connected to each input codec and a corresponding output codec, and a pulse generator is required to generate a pulse at a predetermined different time on each synchronizing line. A circuit which could generate such pulses on each of a large number of synchronizing lines, with a minimum of low cost parts, would aid in minimizing the cost of such telephone switching systems.